Just because someone could predict that you would make a decision doesn't mean you didn't make the decision, it just means you're internally consistent.
Neuronal processes in the brain seem to be build in a way to amplify such tiniest scale difference to generate divergent outcomes from very similar sensory and memory inputs.
But if it's a certainty, with perfect information, then there's no choice to be had. The choice is an illusion.
Without the concept of free will you would have to assume that the cause for the event is originating from something else (a calculation of sorts based on previous states of the universe is a commonly stated alternative).
Free will walks the line of being a physics-based process which is neither to easily deterministic such as stone being heated in the sun or too random such as a radioactive element emitting a radioactive decay at a random/statistical time but still localizes the cause for an event in some assemblage of atoms such as a living being.